The invention relates to a method for digitally modulating a periodic carrier signal of high frequencyxe2x80x94in particular one which may be in the microwave range (a range of frequencies for which propagation phenomena are not negligible)xe2x80x94with a digital modulation signal generally of lower frequency (that to say less than that of the carrier signal), implemented by a modulator circuit comprising at least one modulation cellxe2x80x94in particular a phase-shift keying modulation cellxe2x80x94intended to receive two digital control signals representing at least a partxe2x80x94in particular a componentxe2x80x94of the digital modulation signal. It extends to an electronic modulator circuit designed for implementing such a method.
WO-98.38730 describes an electronic phase-shift keying modulator circuit with distributed structure comprising a distribution line with n phase-shift cells, a plurality of n parallel branches extending from each phase-shift cell and each comprising a switching/modulator circuit, and means for adding in phase the signals coming from the switching/modulator circuits. The number of phase states that can be obtained may be greater than the number of branches when these contain, as the switching/modulator circuit, modulation cells which, in the most elementary embodiment, are cells for modulation by phase-shift keying with two phase states (referred to as BPSK or MDP2) which are intended to receive two complementary digital control signals.
The publication BOVEDA et al.: xe2x80x9cA 0.7-3 GHZ GAAS QPSK/QAM DIRECT MODULATORxe2x80x9d IEEE JOURNAL OF SOLID-STATE CIRCUITS, Vol. 28, No. 12, 1st Dec. 1993, pp. 1340-1349 also describes other examples of applications of BPSK digital modulation cells for carrying out phase and/or amplitude modulations.
In all the known applications of such cells for BPSK digital modulation with two phase states, as described for example by the publication A. PRIMEROSE et al. xe2x80x9cHigh Bit Rate Four, Phase MMIC Remodulation Demodulator and Modulatorxe2x80x9d Proceedings of the GAAS 92 European Gallium Arsenide and related III-V Compounds Applications Symposium 27-29, Apr. 1992, NOORDWIJK, the digital control signals applied to these BPSK cells are, by principle, signals with two complementary states I, I.
The points in the Fresnel plane (phase and/or amplitude states) that can be addressed in these known digital modulation methods and circuits are therefore limited to a value such as 2n in the case of WO-98.38730.
It has, however, become desirable to be able to increase the number of points in the Fresnel plane (phase and/or amplitude states) for a given circuit structure, in order to improve the performances of the modulation for a given cost price, a given size, a given weight and a given energy consumption.
It is therefore an object of the invention to satisfy this requirement in a simple and economical way. It is a particular object thereof to provide a method and a modulator circuit making it possible to address a large number of points (phase and/or amplitude states) with no loss of consumed power and a low energy consumption, so that this circuit and this method can be adapted and used with numerous types of coding and modulation, in numerous applications.
It is a further object of the invention to provide such a method and such a modulator circuit which are advantageously suited to production in microwave monolithic technology (MMIC)xe2x80x94in particular on gallium arsenide GaAsxe2x80x94with excellent precision.
More particularly, it is an object of the invention to permit a simple choice of the phase and/or amplitude states, and to do so at the very moment of its incorporation into the system (and not during the design or fabrication of the integrated circuit itself).
It is more particularly a further object of the invention to provide such a method and such a modulator circuit which are compatible with the constraints of on-board space systems (small size, excellent reliability, low consumption, etc.).
It is also more particularly an object of the invention to provide such a method and such a modulator circuit which are compatible with high bit rate modulation codings such as MCT, which become advantageous when a large number of phase and/or amplitude states are available.
It is also more particularly an object of the invention to provide such a method and such a modulator circuit which can be adapted to any frequency of carrier signals, in particular in the microwave range.
It is also more particularly an object of the invention to provide such a method and such a modulator circuit which, after production, can accept carrier signals as input whose frequency is fixed but which may also be selected from a wide band (for example from the X band for terrestrial observation telemetry, from the K band of multimedia telecommunication satellites, etc.).
It is a further object of the invention to provide a modulation which does not produce substantial perturbations of the line and of the input signal.
To that end, the invention relates to a method for modulating a carrier signal with a digital modulation signal of lower frequency, implemented by a modulator circuit comprising at least one modulation cell intended to receive two digital control signals representing at least a part of the digital modulation signal, wherein, for at least one value of the digital modulation signal, two digital control signals of equal value are applied to at least one given modulation cell. This modulation cell then delivers, for this value of the digital modulation signal (for which the digital control signals have the same value), a signal, referred to as a modulated elementary signal, which is different in amplitude and/or in phasexe2x80x94in particular which is zeroxe2x80x94from that which it delivers when the digital control signals are complementary.
The invention also extends to a circuit designed for implementing the method according to the invention. It therefore also relates to a modulator circuit capable of modulating a carrier signal with a digital modulation signal of lower frequency, comprising at least one modulation cell intended to receive two digital control signals representing at least a part of the digital modulation signal, wherein it is designed so that, for at least one value of the digital modulation signal, at least one given modulation cell receives two digital control signals of equal value.
At least one modulation cell may be of the type with amplitude modulation using a single phase state. Nevertheless, advantageously, and according to the invention, at least one modulation cell is a phase-shift keying modulation cell, the method and the circuit according to the invention carrying out digital modulation at least by phase-shift keying.
Each modulation cell may be a cell for modulation by phase-shift keying of the type with two phase states (referred to as a BPSK or MDP2 cell)xe2x80x94in particular in oppositionxe2x80x94generally having a single amplitude state, but also possibly having two amplitude states. By applying digital control signals of equal value to it, it is given one or two additional phase and/or amplitude state(s). In the event that the phase difference between two states is other than 180xc2x0, and/or in the event that the amplitude of the two states is different, the said modulation cell does not deliver the same modulated elementary signal when the digital control signals that it receives are equal to 0 and when they are equal to 1. Two additional states are hence created for this modulation cell. This modulated elementary signal is not necessarily zero when the digital control signals are of equal value (both equal to 0 or to 1).
Preferably, advantageously and according to the invention, at least one modulation cell receiving two digital control signals of equal value for at least one value of the digital modulation signal is nevertheless designed to deliver a signal, referred to as a modulated elementary signal, which is zero for this value of the digital modulation signal.
Advantageously and according to the invention, at least one modulation cell is furthermore designed to deliver a modulated elementary signal with two phase states in opposition and a single amplitude state when it receives complementary digital control signals, and to deliver a modulated elementary signal which is zero when it receives digital control signals of equal value.
In a method and a circuit according to the invention, each BPSK modulation cell thus delivers a modulated signal that can have three or four distinct valuesxe2x80x94in particular when one of them is the value zeroxe2x80x94, rather than only two values as in the case of a traditional BPSK cell. The various modulated elementary signals of the various modulation cells are combinedxe2x80x94in particular added in phasexe2x80x94in order to form the modulated output signal. It is hence possible to address a larger number of points in the Fresnel plane.
Advantageously, a method according to the invention is one wherein the modulator circuit comprises at least two modulation cells, and wherein, for at least one value of the digital modulation signal, two digital control signals of equal value are applied to at least one modulation cell (this modulation cell preferably delivering a modulated elementary signal which is zero for this value of the digital modulation signal) and two digital control signals of complementary values are applied to at least one other modulation cell (this other modulation cell delivering a modulated elementary signal which is non-zero for this value of the digital modulation signal).
In particular, in the preferred variant of the invention, for a given value of the digital modulation signal, at least one modulation cell delivers a zero modulated elementary signal while at least one other delivers a non-zero modulated elementary signal.
Advantageously, a method according to the invention is also one wherein, for at least one value of the digital modulation signal, two digital control signals of equal value are applied to at least one modulation cell, and wherein, for at least one other value of the digital modulation signal, two digital control signals of complementary values are applied to this (these) modulation cell(s). In particular, a given modulation cell hence does not deliver a zero modulated elementary signal for all the values of the digital modulation signal.
Advantageously and according to the invention, for each modulation cell with two phase states in opposition, when they are of equal value, the digital control signals applied to a given modulation cell are both equal to 0. This avoids the drifts due to technological imperfection of the electronic circuit which would not cancel out the modulated elementary signal perfectly for two digital control signals equal to 1.
Furthermore, traditional BPSK modulation cells (such as those described by WO-98.38730 (FIG. 7) or by the publication A. PRIMEROSE et al. cited above) are impedance-matched well when they receive control signals with complementary states. Nevertheless, such is not the case if the control signals simultaneously take the same value, and in particular when they both take the value 0, especially in the microwave range. However, the inventors have determined that it is actually possible to provide impedance matching means such that the impedance is in practice matched sufficiently for the control signals to be complementary or equal, in so far as the drifts entailed by the impedance matching error are less than, or of the same order as, the technological tolerances.
Advantageously and according to the invention, an input signal coming from the carrier signal is thus delivered to each modulation cell via impedance matching means, which are designed so that the modulation cell is at least substantially impedance-matched both when the digital control signals that it receives are complementary and when they are equal. Advantageously and according to the invention, the impedance matching means comprise, for each modulation cell, a transistor, which receives the input signal and is connected to the modulation cell, and a parallel resistor between the transistor and ground.
Advantageously and according to the invention, a modulator circuit is furthermore used with a distributed structure comprising a plurality of derived branches, each comprising at least one modulation cell intended to receive two digital control signals, each derived branch delivering a signal referred to as a modulated branch-output signal, and the modulated branch-output signals coming from the various derived branches are added in phase in order to form a modulated output signal. In particular, a modulator circuit according to the distributed structure described in WO/98.38730 is used. In the method of the invention, the number of phase states of each BPSK modulation cell is equal to 3 rather than 2, which makes it possible to address 3n points in the Fresnel plane with the modulated output signal, n being the number of branches of the circuit (and not 2n as in WO/98.38730).
The invention extends to a modulator circuit, wherein it is designed for implementing the aforementioned features of the method according to the invention. In particular, a circuit according to the invention advantageously comprises, upstream of each modulation cell, impedance matching means which supply the modulation cell with an input signal coming from the carrier signal and which are designed so that the modulation cell is at least substantially impedance-matched both when the digital control signals that it receives are complementary and when they are equal.
The invention also relates to a method and to a modulator circuit wherein there are all or some of the features mentioned above or below in combination.